The following paragraphs are not an admission that anything discussed in them is prior art or part of the knowledge of persons skilled in the art.
In water cooled nuclear power plants, following a loss of coolant accident (LOCA), water and insulation debris dislodged at the break location may accumulate in the sump area. After the initial emergency water injection phase, the sump water may be re-circulated back to the reactor core as part of the emergency core cooling (ECC) system to prevent fuel melt. The debris in the sump water may be filtered by ECC strainers so that the debris will not deposit in the reactor core, which may result in flow blockages and buildup of thermal resistance layers on fuel elements, and may cause the fuel to overheat and melt. Although ECC strainers may catch almost all debris on the strainer surface, a small amount of debris may go through the strainer holes and into the reactor core.
Deposition of the debris in the reactor core is considered a safety issue, because the nuclear fuel keeps producing nuclear energy even after the reactor is safely shutdown (through radioactive decays of unstable isotopes produced in the core). For various water cooled reactors (e.g., PWRs, BWRs and CANDUs), there are either established safe limits of how much debris is allowed to bypass the strainers, or such limits are being developed. Although typical debris includes various types of particulates and fibres, the bypass limits may be set for the amount of fibre only. It is, hence, a functional requirement that ECC strainers be able to filter fibres so that these fibre bypass limits are not exceeded.
There are existing methods to quantify fibre bypass. These methods include collecting water samples at various times downstream of the strainer. The samples may be useful to quantify the transient evolution of the downstream fibre concentration. The samples may be processed by filtering the debris using a fine filter paper, drying the filter paper, and weighing the increase in the weight of filter paper (this technique may be referred to as the “weighing technique”). The overall fibre bypass may then be obtained by integrating the weights of the fibres obtained from individual samples over time. Alternatively, a very fine downstream filter may be used to capture all fibres that bypassed the strainer surface (this technique may be referred to as the “downstream filter technique”). This technique may be more accurate as it gives the total weight of the captured fibres, but may get plugged in some tests that use particulates.
In an actual ECC system, the short-term, high-fibre-concentration flow may last a few hours (corresponding to a few flow turnovers). The requirement for the allowable overall fibre bypass may be defined for one month of ECC operation after the start of the ECC recirculation system. The movement of individual fibres from the debris bed and occasional local collapse of the debris bed may provide a steady supply of a small amount of fibre to the downstream. Hence, even if the downstream fibre concentrations are small, a significant amount of fibre may bypass the strainers after the initial high-fibre-concentration transient. The weighing technique may be a good way of quantifying debris bypass shortly after the ECC recirculation pumps are engaged and a significant amount of debris may be bypassing the clean strainer surface. Most of this debris may end up accumulating on the strainer surface, creating a mat of fibres that may eventually behave like a layer of fine filter on top of the strainer surface. As a result, the fibre concentration downstream of the strainer may eventually become sufficiently small to render the weighing technique impractical (because the weight of the captured fibre becomes a very small fraction of the combined weight of the filter and fibre after filtering).
Hence, other techniques, suitable to quantify a small amount of fibre in water mixed with particulates, are desirable.